1. Field of the Invention.
The present invention relates generally to auxiliary hydraulic systems for attachments mounted to skid steer loaders. In particular, the present invention is an electrically controlled auxiliary hydraulic system.
2. Description of the Prior Art.
Skid steer loaders are compact, highly maneuverable vehicles which are widely used in a variety of applications. These vehicles typically include a rear mounted engine which drives several hydraulic pumps. A first variable displacement hydraulic pump is fluidly coupled to a first hydraulic motor on the left side of the vehicle, while a second variable displacement hydraulic pump is coupled to a second motor on the right side of the vehicle. Wheels on the left and right sides of the loader are driven by their respective motors through chain and sprocket linkages. An operator seated within an operator compartment controls the motion of the vehicle by actuating a pair of steering levers which are linked to the variable displacement hydraulic pumps. The extent to which each lever is pushed in the forward direction controls the amount of fluid supplied in a first direction to its respective hydraulic motor, and therefore the speed at which the wheels on that side of the vehicle will rotate. Similarly, the extent to which a lever is pulled in the reverse direction will control the speed at which the wheels on that side of the vehicle are rotated in the reverse direction.
Skid steer loaders also typically include a boom assembly formed by a pair of lift arms pivotally mounted to the main frame. Attachments are pivotally mounted to the front of the lift arms by means of an attachment mount. A separate hydraulic system is used to actuate the boom assembly. Hydraulic lift cylinders which drive the lift arms with respect to the main frame and a tilt cylinder which drives the attachment mount with respect to the lift arms are supplied with hydraulic fluid by a constant displacement implement pump. A pair of foot pedals in the front of the operator's compartment are mechanically linked to spool valves and actuated by an operator to control the flow of hydraulic fluid to the lift and tilt cylinders.
Attachments such as an auger, grapple, sweeper, landscape rake, snow blower or backhoe which include their own hydraulic drive motor are sometimes mounted to the boom assembly. An auxiliary hydraulic system is used to control the flow of hydraulic fluid between the implement pump and the hydraulic motor of the front mounted attachment. In one known system the flow of hydraulic fluid to the motor is controlled by an auxiliary spool valve through actuation of one of the steering levers. The lever is normally biased to a central position. Pushing the lever to the left strokes the auxiliary valve in a first direction, thereby causing hydraulic fluid to flow to the front mounted attachment in a first or forward direction. Pushing the steering lever in the opposite direction (i.e. to the right) strokes the auxiliary valve in such a manner as to supply fluid in a second or reverse direction. The lever is mounted to the floor by an over center pivot arrangement so that it can be latched in the rightward direction, thereby permitting continued flow of fluid in the forward direction.
Feller bunchers are compact and maneuverable vehicles commonly used to harvest trees. These vehicles include a front mounted grapple and shear attachment which is powered by a hydraulic motor. An electrically actuated hydraulic valve controls the flow of fluid to the attachment-driving motor. The operator uses switches mounted to the steering levers to handles of actuate the valve. The grapple is driven in a first direction (e.g., closed) as long as a first switch is pressed, and in a second direction (e.g., opened) while a second switch is pressed.
Attachments such as scarifiers or stabilizers which also include hydraulic motors are sometimes mounted to the rear of the loader. These rear mounted attachments are also supplied with hydraulic fluid from the implement pump by an auxiliary hydraulic system.
In one skid steer loader the valves used to control the lift cylinders, tilt cylinders, and front mounted auxiliary are valves in a four-spool series valve block. The fourth spool valve of the block can be coupled to a rear mounted auxiliary by hoses, and actuated by a mechanical linkage through the left steering control lever in a manner similar to that of the front mounted auxiliary. In still other embodiments, a separate spool valve is mounted within the vehicle and coupled to a lever by a mechanical linkage. This spool valve is then used with any rear mounted attachment the loader may be carrying.
Alternatively, a manually actuated diverter valve is sometimes used to route hydraulic fluid from the front mounted attachment to the rear mounted attachment. The spool valve used to control the front mounted attachment can then also be used to control the rear mounted attachment.
When a skid steer loader is equipped with a front or rear mounted attachment in which most of the work is performed by the hydraulic motor of the attachment, overall system performance could be enhanced if a larger percentage of the engine power were available from the hydraulic system. However, since the predominant use of skid steer loaders is with attachments not equipped with hydraulic motors, and many of the attachments that do have motors require less power than is normally available, little or no increase in hydraulic power is generally required. System pressure, which is regulated by relief valves, is therefore kept lower than that which could be accommodated by the system. Operation in this manner helps prolong the life of seals and other system components.
It is evident that there is a continuing need for improved skid steer loader hydraulic systems. A hydraulic system which is capable of accommodating both front and rear mounted attachments is needed. A system capable of controlling the amount of power available to the attachments would also be desirable. The hydraulic system must of course be reliable and convenient to use.